Conventional rotating bits used for drilling mining blast holes or geothermal wells normally employ three cone-shaped rolling cutters mounted on a journal arm or spindle. The cutters are mounted on the journal arm for rotational movement using rollers and balls to produce an anti-friction bearing. The drill bit is rotated by a drill string and a heavy load is brought to bear upon the cutters which rotate at the bottom of the borehole producing chips and other debris.
In order to cool the anti-friction bearings, air is pumped down the drill string through the journal arms into the anti-friction bearings. The air flows through the passageways between the rollers and out of the mouth of the cone-shaped cutters. Some provision must be made to allow the air to escape from the mouth of the cutter to the borehole while at the same time avoiding the incursion of borehole cuttings and other debris into the anti-friction bearing. Should such incursion take place, the bearings will wear prematurely and the useful life of the bit will be reduced.
In U.S. Pat. No. 3,921,735, issued to me on Nov. 25, 1975 entitled "Rotary Rock Bit With Cone Mouth Air Screen" assigned to Dresser Industries, Inc., a drill bit is described which provides a clearance between the shoulder of the cone mouth and the journal arm which is large enough to permit air to escape to the borehole. The clearance extends around the entire circumference of the journal arm so that air passing through all of the passages between the rollers escapes around the entire periphery of the cone mouth. In order to provide a barrier to the cuttings and other debris from entering the anti-friction bearings, a screen was inserted into the clearance between the cone mouth and the journal arm which allows the passage of air, but blocks the cuttings. The useful life of the screen material in this case becomes one of the determinative factors in the useful life of the bit, since failure of the screen will allow incursion of cuttings and debris through the entire circumference of the cone mouth clearance.
In another U.S. Pat. No. 4,688,651 issued to me on Aug. 25, 1987 entitled "Cone Mouth Debris Exclusion Shield" assigned to Dresser Industries, Inc., another air vent is disclosed. Once again a clearance is provided between the cone mouth and the journal arm around the entire circumference of the cutter and journal arm. In the upper or dome region of the bit a shield is inserted into the clearance between the cone mouth and journal arm. This shield, which is in the form a spring steel element, closes off the clearance between the cutter mouth and journal arm in the dome region of the bit causing the air to be vented out of the bottom half or shirttail region of the bit. The shield adds an additional component to the bit and additional manufacturing operations for mounting the shield.
Another U.S. Pat. No. 4,421,184 issued on Dec. 20, 1983 to John M. Mullins entitled "Rock Bit With Improved Shirttail Ventilation", assigned to Hughes Tool Company discloses a rock bit with a clearance between the cone mouth and journal arm which allows some air to be discharged in the dome region of the bit, but most of the air is discharged through a slot formed in the shirttail region of the bit. The slot exposes the passageways between the rollers, and the rollers themselves to direct view from the borehole.
Another commonly used air vent for a drilling bit is disclosed below in the Detailed Description Of The Drawings. In this bit a narrow clearance is established between the shoulder of the cutter cone mouth and a surface of the journal arm called the last machined surface, which is the surface on the journal arm formed during the last machining operation and closest to the region where the journal arm joins the leg of the bit. This clearance is narrow and substantially blocks the venting of air from the cone mouth. A groove is cut near the dome region of the bit into the last machined surface. The width of the groove is sufficient to reach the raceway in which the rollers travel thereby opening a number of passageways between the rollers for venting around the cone mouth into the borehole. This form of vent allows most of the cone mouth and last machined surface of the journal arm to block off the incursion of borehole cuttings and other debris into the anti-friction bearings. However, due to the location of the vent near the dome area, the bit cuttings and debris can plug or clog a portion of the air vent reducing the amount of cooling air, and also permit cuttings and debris to drop into the groove and enter the anti-friction bearings.